Systems and methods for alkaline earth production

ABSTRACT

Hydrometallurgical systems, methods, and compositions are described in which amine-based lixiviants are utilized in substoichiometric amounts to recover alkaline earths from raw or waste materials. The lixiviant can be regenerated and recycled for use in subsequent iterations of the process or returned to a reactor in a continuous process. Extraction of the alkaline earth from the raw material and precipitation of the extracted alkaline earth is performed in the same reactor and essentially simultaneously.

This application is a continuation-in-part of U.S. patent application Ser. No. 14/553,640, filed Nov. 25, 2014, which is a continuation-in-part of U.S. patent application Ser. No. 14/073,503, filed Nov. 6, 2013, and claims the benefit of priority of U.S. Provisional Application No. 61/985,036, filed Apr. 28, 2014, and U.S. Provisional Application No. 61/908,590, filed Nov. 25, 2013. These and all other referenced extrinsic materials are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The field of the invention is hydrometallurgy, particularly as it is related to the extraction or recovery of alkaline earth elements.

BACKGROUND

The following description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.

All publications identified herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.

There is a long-standing need to efficiently and cost-effectively recover commercially valuable metals from low yield sources, such as mine tailings.

Historically, it has been especially desirable to recover alkaline earth elements. Alkaline earth elements, also known as beryllium group elements, include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and radium, (Ra), which range widely in abundance. Applications of these commercially important metals also vary widely, and include uses as dopants in electronic components, structural materials, and in the production foods and pharmaceuticals.

Methods of isolating of one member of the alkaline earth family, calcium, from minerals such as limestone, have been known since ancient times. In a typical process limestone is calcined or otherwise roasted to produce calcium oxide (CaO), or quicklime. This material can be reacted with water to produce calcium hydroxide (Ca(OH)₂), or slaked lime. Calcium hydroxide, in turn, can be suspended in water and reacted with dissolved carbon dioxide (CO₂) to form calcium carbonate (CaCO₃), which has a variety of uses. Approaches that have been used to isolate other members of this family of elements often involve the production of insoluble hydroxides and oxides using elevated temperatures or strong acids. Such approaches, however, are not suitable for many sources of alkaline earth elements (such as steel slag), and are not sufficiently selective to be readily applied to mixtures of alkaline earth elements.

Hydrometallurgy can also been used to isolate metals from a variety of minerals, ores, and other sources. Typically, ore is crushed and pulverized to increase the surface area prior to exposure to the solution (also known as a lixiviant). Suitable lixiviants solubilize the desired metal, and leave behind undesirable contaminants. Following collection of the lixiviant, the metal can be recovered from the solution by various means, such as by electrodeposition or by precipitation from the solution.

Previously known methods of hydrometallurgy have several problems. Identification of lixiviants that provide efficient and selective extraction of the desired metal or metals has been a significant technical barrier to their adoption in the isolation of some metals. Similarly, the expense of lixiviant components, and difficulties in adapting such techniques to current production plants, has limited their use.

Approaches have been devised to address these issues. United States Patent Application No. 2004/0228783 (to Harris, Lakshmanan, and Sridhar) describes the use of magnesium salts (in addition to hydrochloric acid) as a component of a highly acidic lixiviant used for recovery of other metals from their oxides, with recovery of magnesium oxide from the spent lixiviant by treatment with peroxide. Such highly acidic and oxidative conditions, however, present numerous production and potential environmental hazards that limit their utility. In an approach disclosed in U.S. Pat. No. 5,939,034 (to Virnig and Michael), metals are solubilized in an ammoniacal thiosulfate solution and extracted into an immiscible organic phase containing guanidyl or quaternary amine compounds. Metals are then recovered from the organic phase by electroplating.

A similar approach is disclosed in U.S. Pat. No. 6,951,960 (to Perraud) in which metals are extracted from an aqueous phase into an organic phase that contains an amine chloride. The organic phase is then contacted with a chloride-free aqueous phase that extracts metal chlorides from the organic phase. Amines are then regenerated in the organic phase by exposure to aqueous hydrochloric acid. Application to alkaline earth elements (for example, calcium) is not clear, however, and the disclosed methods necessarily involve the use of expensive and potentially toxic organic solvents.

In a related approach, European Patent Application No. EP1309392 (to Kocherginsky and Grischenko) discloses a membrane-based method in which copper is initially complexed with ammonia or organic amines. The copper:ammonia complexes are captured in an organic phase contained within the pores of a porous membrane, and the copper is transferred to an extracting agent held on the opposing side of the membrane. Such an approach, however, requires the use of complex equipment, and processing capacity is necessarily limited by the available surface area of the membrane.

Methods for capturing CO₂ could be used to recover alkaline earth metals, but to date no one has appreciated that such could be done. Kodama et al. (Energy 33 (2008), 776-784) discloses a method for CO₂ capture using a calcium silicate (2CaO.SiO₂) in association with ammonium chloride (NH₄Cl). This reaction forms soluble calcium chloride (CaCl₂), which is reacted with carbon dioxide (CO₂) under alkaline conditions to form insoluble calcium carbonate (CaCO₃) and release chloride ions (Cl−).

Kodama et al. uses clean forms of calcium to capture CO₂, but is silent in regard to the use of other alkaline earth elements in this chemistry. That makes sense from Kodoma et al.'s disclosure, which notes that a high percentage (approximately 20%) of the NH₄Cl used is lost in the disclosed process, requiring the use of additional equipment to capture ammonia vapor. This loss results in significant process inefficiencies, and raises environmental concerns. Japanese Patent Application No. 2005097072 (to Katsunori and Tateaki) discloses a similar method for CO₂ capture, in which ammonium chloride (NH₄Cl) is dissociated into ammonia gas (NH₃) and hydrochloric acid (HCl), the HCl being utilized to generate calcium chloride (CaCl₂) that is mixed with ammonium hydroxide (NH₄OH) for CO₂ capture.

International Application WO 2012/055750 (to Tavakkoli et al) discloses a method for purifying calcium carbonate (CaCO₃), in which impure CaCO₃ is converted to impure calcium oxide (CaO) by calcination. The resulting CaO is treated with ammonium chloride (NH₄Cl) to produce calcium chloride (CaCl₂), which is subsequently reacted with high purity carbon dioxide (CO₂) to produce CaCO₃ and NH₄Cl, with CaCO₃ being removed from the solution by crystallization with the aid of seed crystals. Without means for capturing or containing the ammonia gas that would result from such a process, however, it is not clear if the disclosed method can be adapted to an industrial scale.

Thus, there is still a need for a hydrometallurgical method that provides simple and economical isolation of metal hydroxide forming species.

SUMMARY OF THE INVENTION

The inventive subject matter provides hydrometallurgical systems, methods, and compositions in which amine-based lixiviants are utilized in substoichiometric amounts to recover alkaline earths from raw or waste materials. The lixiviant can be regenerated and recycled for use in subsequent iterations of the process or returned to a reactor in a continuous process. Extraction of the alkaline earth from the raw material and precipitation of the extracted alkaline earth is performed in the same reactor and essentially simultaneously.

One embodiment of the inventive concept is a method for recovering an alkaline earth (for example, calcium) from an alkaline earth-bearing raw material or waste product. In such a method the raw material (for example, a steel slag), an amine containing lixiviant, and a precipitant (i.e. a compound that reacts with an alkaline earth released from the raw material to form a precipitate, for example a gas that contains CO₂) are brought into contact in reactor. The lixiviant is provided in substoichiometric amounts relative to the amount of alkaline earth available in the raw material. The subsequent series of reactions produces an alkaline earth precipitate and an extracted raw material, and in the process regenerates the lixiviant species. The alkaline earth precipitate and the extracted raw material are separated, and some or all of the regenerated lixiviant is returned to the reactor.

The alkaline earth precipitant and the extracted raw material can be separated on the basis of particle size/diameter, particle density, and/or differential magnetic properties. Suitable separators include settling tanks filters, centrifugal separators, and magnetic separators. For example the reactor, or a portion thereof, can be configured as a settling tank. In some embodiments the separation is performed on a batch basis. In other embodiments the separation is performed on a continuous basis.

In some embodiments of the inventive concept, the raw material, the lixiviant, and/or the precipitant are added to the reactor in an essentially continuous manner. Similarly, in other embodiments of the inventive concept the separation process is performed in an essentially continuous manner.

In still other embodiments of the inventive concept the extracted raw material, which is relatively enriched in non-extracted elements following extraction of the alkaline earth component, is not discarded, but rather is retained and subjected to further processing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an example of a method of the inventive concept in which calcium is recovered from a sample, using an organic amine chloride lixiviant that is regenerated.

FIG. 2 schematically depicts a method of the inventive concept, in which a alkaline earth elements is recovered from a sample using a lixiviant, which is regenerated.

FIG. 3 schematically depicts another method of the inventive concept, in which different alkaline earth elements are recovered in a stepwise fashion.

FIG. 4 schematically depicts an alternative embodiment of the inventive concept, in which different alkaline earth elements are recovered in a stepwise manner.

FIG. 5 illustrates the composition of a steel slag.

FIG. 6 schematically depicts processing of a steel slag by a method of the inventive concept.

FIG. 7A-D show the results of systems, methods, and compositions of the inventive concept. FIG. 7A shows pH changes over time as an alkaline earth element is extracted from a low grade source using an organic amine lixiviant. FIG. 7B shows pH changes over time as an alkaline earth element is extracted from a low grade source using a different organic amine lixiviant. FIG. 7C shows pH changes over time as an extracted alkaline earth element is recovered through the use of a precipitant. FIG. 7D is a photomicrograph of a precipitated calcium carbonate product of systems, methods, and compositions of the inventive concept.

FIG. 8A-E show a prior art commercial plant and a commercial plant adapted to utilize a process of the inventive concept. FIG. 8A depicts components and material flow in a prior art processing plant. FIG. 8B depicts components and material flow in a processing plant that has been modified to perform a process of the inventive concept. FIG. 8C depicts the use of multi-effect evaporators in a modified plant such as that shown in FIG. 8B. FIG. 8D and FIG. 8E schematically depict exemplary mass balances and reaction conditions for processes of the inventive concept applied to steel slag and to lime, respectively.

FIGS. 9A and 9B schematically depict a single-step method of the inventive concept, wherein extraction of the alkaline earth and precipitation are not segregated. FIG. 9A depicts a method that utilizes a lixiviant. FIG. 9B depicts a method in that does not include an added lixiviant.

FIGS. 10A and 10B schematically depict an alternative single-step method of the inventive concept, wherein extraction of the alkaline earth and precipitation are not segregated. FIG. 10A depicts a method that utilizes a lixiviant. FIG. 10B depicts a method in that does not include an added lixiviant.

FIG. 11 schematically depicts an alternative embodiment of a method of the inventive process, wherein extraction of the alkaline earth and precipitation are partially segregated.

FIG. 12A and FIG. 12B show exemplary results from one-step and two-step methods of alkaline earth recovery. FIG. 12A shows recovery of calcium in the form of calcium carbonate from two different steel slags. FIG. 12B shows the increase in yield of calcium carbonate from a one-step process compared to a prior art two-step process from two different steel slags as a function of the particle size of the respective steel slags.

FIG. 13 shows the distribution of different products from a single-step method of the inventive concept as a function of the particle size of the raw material.

FIG. 14 shows the yield of calcium in the form of calcium carbonate at different ratios of amine lixiviant to calcium available in the raw material.

FIG. 15 shows the distribution of different products from a single-step method of the inventive concept at different ratios of amine lixiviant to calcium available in the raw material.

FIG. 16 shows exemplary results from pH monitoring during a prior art two-step method.

FIG. 17A and FIG. 17B show exemplary results from pH monitoring during a single-step method of the inventive concept. FIG. 17A shows the change in pH on suspension of raw material (1), addition of lixiviant (2), first addition of CO₂ (3), cessation of CO₂ (4), second addition of CO₂ (5), and termination (6). FIG. 17B shows a detail of pH changes during a CO₂ addition at different lixiviant:alkaline earth ratios and with different sources of raw material.

FIG. 18 depicts the increase in the amount of steel slag raw material required to produce one ton of calcium carbonate by the traditional two-step method over the amount required by a one-step method of the inventive concept.

FIG. 19 depicts a table showing the composition of different steel slags.

FIG. 20 depicts a table showing experimental conditions used in two-step and one-step processes.

DETAILED DESCRIPTION

The inventors have discovered a hydrometallurgical method for the recovery of alkaline earth elements (i.e., alkaline earth metals), such as members of the alkaline earth family, through the use of lixiviants that include organic amines. The inventors have determined that such amine-based lixiviants can be regenerated using carbon dioxide. Surprisingly, this regeneration permits extraction of an alkaline earth from a raw material and precipitation of the extracted alkaline earth, for example in the form of a carbonate, in the same reactor and essentially simultaneously, with differences between the physical properties of the carbonate salt produced and the extracted raw material permitting separation by simple physical means.

Throughout the following discussion, numerous references will be made regarding lixiviants. A lixiviant should be understood to be a chemical entity that has the ability to selectively extract metals or metal ions from inorganic or organic solids in an aqueous or other solvent mixture. Similarly, a precipitant should be understood to be a chemical entity that has the ability to form a precipitate that includes such extracted metals or metal ions.

The following discussion provides many exemplary embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.

In some embodiments, the numbers expressing quantities of ingredients, properties such as concentration, reaction conditions, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The numerical values presented in some embodiments of the invention may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements. Similarly, unless the context dictates the contrary all ranges set forth herein should be interpreted as being inclusive of their endpoints and open-ended ranges should be interpreted to include only commercially practical values. Similarly, all lists of values should be considered as inclusive of intermediate values unless the context indicates the contrary.

As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.

The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value with a range is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.

Embodiments of the inventive process can include at least one compound of the general composition depicted as in Compound 1 for use with any source of material that contains one or more a form(s) of a alkaline earth (AE) hydroxide forming species, that can be hydrated to form AE(OH)x or other hydrated species that would react with lixiviants of the form found in Equation 1. Alternatively, alkaline earth elements can be presented as oxides, for example calcium oxide (CaO), that can form hydroxides on reaction with water. Such hydrated forms may be present in the material as it is obtained from nature or can be introduced by processing (for example through treatment with a base, hydration, or by oxidation), and can be stable or transient. Selective extraction of a desired alkaline earth can be based on the presence of a metal hydroxide that has a stronger basicity than the organic amine-based lixiviants used in the extraction process.

Organic amines of the inventive concept have the general formula shown in Compound 1, where N is nitrogen, H is hydrogen, R₁ to R₃ can be an organic (i.e. carbon-containing) group or H, and X is a counterion (i.e., a counter anion). Ny,R₁,R₂,R₃,H—Xz  Compound 1 Suitable counterions can be any form or combination of atoms or molecules that produce the effect of a negative charge. Counterions can be halides (for example fluoride, chloride, bromide, and iodide), anions derived from mineral acids (for example nitrate, phosphate, bisulfate, sulfate, silicates), anions derived from organic acids (for example carboxylate, citrate, malate, acetate, thioacetate, propionate and, lactate), organic molecules or biomolecules (for example acidic proteins or peptides, amino acids, nucleic acids, and fatty acids), and others (for example zwitterions and basic synthetic polymers). For example, monoethanolamine hydrochloride (MEA.HCl, HOC₂H₄NH₃Cl) conforms to Compound 1 as follows: one nitrogen atom (N₁) is bound to one carbon atom (R₁=C₂H₅O) and 3 hydrogen atoms (R₂, R₃ and H), and there is one chloride counteranion (X₁=Cl−). Compounds having the general formula shown in Compound 1 can have a wide range of acidities, and an organic amine of the inventive concept can be selected on the basis of its acidity so that it can selectively react with one or more alkaline earth metal salts or oxides from a sample containing a mixture of alkaline earth metal salts or oxides. Such a compound, when dissolved in water or another suitable solvent, can (for example) effectively extract the alkaline earth element calcium presented in the form calcium hydroxide in a suitable sample (e.g. steel slag). Equation 1 depicts a primary chemical reaction in extracting an insoluble alkaline earth (AE) salt (in this instance a hydroxide salt) from a matrix using an organic amine cation (OA-H+)/counterion (Cl−) complex (OA-H+/Cl−) as a lixiviant. Note that the OA-H+/Cl− complex dissociates in water into OA-H+ and Cl−. AE(OH)₂(solid)+2OA-H+(aq)+2Cl−(aq)→AECl₂(aq)+2OA(aq)+2H₂O  Equation 1

The counterion (Cl−) is transferred from the organic amine cation (OA-H+) to the alkaline earth salt to form a soluble alkaline earth/counterion complex (AECl₂), uncharged organic amine (OA), and water. Once solubilized the alkaline earth/counterion complex can be recovered from solution by any suitable means. For example, addition of a second counterion (SC) in an acid form (for example. H₂SC), which reacts with the alkaline earth cation/counterion complex to form an insoluble alkaline earth salt (AESC), can be used to precipitate the extracted alkaline earth from supernatant and release the counterion to regenerate the organic amine cation/counterion pair, as shown in Equation 2. AECl₂(aq)+2OA(aq)+H₂SC→AESC salt(solid)+2OA+(aq)+2Cl−  Equation 2

Examples of suitable second counterions include polyvalent cations, for example carbonate (which can be supplied as CO₂), sulfate, sulfite, chromate, chlorite, and hydrogen phosphate.

Alternatively, pH changes, temperature changes, or evaporation can be used to precipitate the solubilized alkaline earth. In some embodiments, the alkaline earth element can be recovered by electrodeposition processes, such as electrowinning or electrorefining. In other embodiments of the inventive concept the solubilized alkaline earth element can be recovered by ion exchange, for example using a fixed bed reactor or a fluidized bed reactor with appropriate media.

In preferred embodiments of the inventive concept, an alkaline earth element is recovered by precipitation through reaction of the alkaline earth/lixiviant mixture with carbon dioxide (CO₂), which advantageously regenerates the lixiviant as shown below. It should be appreciated that the process of recovering the alkaline earth element can be selective, and that such selectivity can be utilized in the recovery of multiple alkaline earth elements from a single source as described below.

The organic amine cation/counterion complex can be produced from the uncharged organic amine to regenerate the OA-H+/Cl− lixiviant, for example using an acid form of the counterion (H—Cl), as shown in Equation 3. OA(aq)+H−Cl(aq)→OA-H+(aq)+Cl−  Equation 3 In some embodiments of the inventive concept the reaction described in Equation 3 can be performed after the introduction of an uncharged organic amine to a source of an alkaline earth element, with the lixiviant being generated afterwards by the addition of an acid form of the counterion. This advantageously permits thorough mixing of the alkaline earth source with a lixiviant precursor prior to initiating the reaction.

Organic amines suitable for the extraction of alkaline earth elements (for example from calcium containing or, steel slag, and other materials) can have a pKa of about 7 or about 8 to about 14, and can include protonated ammonium salts (i.e., not quaternary). Examples of suitable organic amines for use in lixiviants include weak bases such as ammonia, nitrogen containing organic compounds (for example monoethanolamine, diethanolamine, triethanolamine, morpholine, ethylene diamine, diethylenetriamine, triethylenetetramine, methylamine, ethylamine, propylamine, dipropylamines, butylamines, diaminopropane, triethylamine, dimethylamine, and trimethylamine), low molecular weight biological molecules (for example glucosamine, amino sugars, tetraethylenepentamine, amino acids, polyethyleneimine, spermidine, spermine, putrecine, cadaverine, hexamethylenediamine, tetraethylmethylenediamine, polyethyleneamine, cathine, isopropylamine, and a cationic lipid), biomolecule polymers (for example chitosan, polylysine, polyornithine, polyarginine, a cationic protein or peptide), and others (for example a dendritic polyamine, a polycationic polymeric or oligomeric material, and a cationic lipid-like material), or combinations of these. In some embodiments of the inventive concept the organic amine can be monoethanolamine, diethanolamine, or triethanolamine, which in cationic form can be paired with nitrate, bromide, chloride or acetate anions. In other embodiments of the inventive concept the organic amine can be lysine or glycine, which in cationic form can be paired with chloride or acetate anions. In a preferred embodiment of the inventive concept the organic amine is monoethanolamine, which in cationic form can be paired with a chlorine anion.

Such organic amines can range in purity from about 50% to about 100%. For example, an organic amine of the inventive concept can have a purity of about 50%, about 55%, about 60%, about 65%, about 70%, about 75%, about 80%, about 85%, about 90%, about 95%, about 97%, about 99%, or about 100%. In a preferred embodiment of the inventive concept the organic amine is supplied at a purity of about 90% to about 100%. It should be appreciated that organic amines can differ in their ability to interact with different members of the alkaline earth family and with contaminating species, and that such selectivity can be utilized in the recovery of multiple alkaline earths as described below.

Inventors further contemplate that zwitterionic species can be used in suitable lixiviants, and that such zwitterionic species can form cation/counterion pairs with two members of the same or of different molecular species. Examples include amine containing acids (for example amino acids and 3-aminopropanoic acid), chelating agents (for example ethylenediamine-tatraacetic acid and salts thereof, ethylene glycol tetraacetic acid and salts thereof, diethylene triamine pentaacetic acid and salts thereof, and 1,2-bis(o-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid and salts thereof), and others (for example betaines, ylides, and polyaminocarboxylic acids).

Organic amines for use in lixiviants can be selected to have minimal environmental impact. The use of biologically derived organic amines, such as glycine, is a sustainable practice and has the beneficial effect of making processes of the inventive concept more environmentally sound. In addition, it should be appreciated that some organic amines, such as monoethanolamine, have a very low tendency to volatilize during processing. In some embodiments of the inventive concept an organic amine can be a low volatility organic amine (i.e., having a vapor pressure less than or equal to about 1% that of ammonia under process conditions). In preferred embodiments of the inventive concept the organic amine is a non-volatile organic amine (i.e., having a vapor pressure less than or equal to about 0.1% that of ammonia under process conditions). Capture and control of such low volatility and non-volatile organic amines requires relatively little energy and can utilize simple equipment. This reduces the likelihood of such low volatility and non-volatile organic amines escaping into the atmosphere and advantageously reduces the environmental impact of their use.

An example of an application of the inventive concept is in the isolation of insoluble calcium hydroxide, using an ammonium chloride containing lixiviant or, alternatively, in the absence of an added lixiviant. Any source that contains a basic form of calcium can be suitable for use in a process of the inventive concept, for example ores containing alkaline earth oxides and/or hydroxide calcined ores containing alkaline earth oxides and/or hydroxides, partially calcined ores containing alkaline earth oxides and/or hydroxides, steel slag, fly ash, cement kiln dust, ash, shale ash, acetylene catalyst waste, dolime, lime, low-grade lime, ash from municipal waste, and calcium hydroxide. In some embodiments of the inventive concept a calcium source can be selected on the basis of high calcium content per unit mass with high levels of contamination, for example low grade lime or dolomitic lime. In other embodiments of the inventive concept, calcium can be recovered from lime, for example low grade lime. Equation 4 represents a reaction that takes place on contacting calcium hydroxide (Ca(OH)₂))-containing steel slag with an ammonium chloride lixiviant. Ca(OH)₂(solid)+2NH₄+(aq)+2Cl−(aq)→CaCl₂(aq)+2NH₃(aq)+2H₂O  Equation 4 Calcium is extracted from the slag as soluble calcium chloride (CaCl₂), with the generation of uncharged ammonia (NH₃) and water.

A soluble alkaline earth salt, for example calcium chloride and the soluble ammonia from Equation 4 (or soluble ammonium ion if the reaction is metal oxide/hydroxide limited) can easily be separated from the insoluble solid residue, for example by filtration. Once separated, the soluble aqueous fraction can be used as-is if the target process can tolerate the small quantity of ammonia or ammonium chloride. Alternatively, the solution can be further processed as needed. In a preferred embodiment of the inventive concept the lixiviant is regenerated and the alkaline earth calcium is recovered as an insoluble salt through the addition of carbon dioxide (CO₂), as shown in Equation 5. It should be appreciated that aqueous CO₂ can be in the form of ionized carbonic acid (i.e., 2H+ plus CO₃2−). CaCl₂(aq)+2NH₃(aq)+CO₂(aq)+H₂O→CaCO₃(solid)+2NH₃(aq)++2Cl−(aq)  Equation 5

It should be appreciated that systems, methods, and compositions of the inventive concept can also be used to selectively extract and/or refine a desired alkaline earth element (such as calcium) from an ore containing other contaminants, for example other alkaline earth elements. By using the lixiviants described herein, one skilled in the art can exploit the varying degrees of basicity associated with each alkaline earth element, and choose a lixiviant of corresponding acidity to achieve selective extraction.

As noted above, in many instances the use of a low volatility and/or non-volatile lixiviant is desirable. An example of such a process of the inventive concept is the extraction of calcium (Ca) from an ore using a non-volatile organic amine, such as monoethanolamine hydrochloride, as shown in FIG. 1. FIG. 1 depicts a process 100 in which, in an initial step 105, a tank or other suitable arrangement includes an aqueous solution of an organic amine 110 (in this instance monoethanolamine) and a mud or slurry 120 containing calcium hydroxide (Ca(OH)₂ and unwanted contaminants (CONT). The solvent used can be any protic or highly polar solvent that can support the solvation of calcium salts in large amounts. Examples of suitable solvents include water, glycerol, and water glycerol mixtures. The amount of organic amine can be optimized for efficient alkaline earth extraction and minimal use of organic amine. For example, in some embodiments the amount of a monovalent organic amine can be selected to be at least about twice that of the available alkaline earth element on a molar basis. In some embodiments of the inventive concept the amount of a monovalent organic amine can be selected to be at least about 2.1 times to about 2.05 times that of the available alkaline earth element. Amounts of organic amines with greater charges can be adjusted accordingly (for example, an organic amine that forms a divalent cation can be used in at least a 1:1 ratio with the available alkaline earth element). Alternatively, and as described in further detail below, regeneration of the lixiviant species can permit the lixiviant species to be utilized in sub-stoichiometric amounts.

Reaction conditions can also be optimized by adjusting the surface area available for the reaction. Particle size of the calcium containing raw material can be reduced prior to exposure to lixiviant, for example by grinding, milling, or sifting. In some embodiments of the inventive concept the particle size can range from about 0.05 mm to about 1 mm. In other embodiments of the inventive concept the particle size can range from about 0.05 to about 0.25 mm. In a preferred embodiment the particle size can range from about 0.05 mm to about 0.125 mm.

The calcium content of the solution can also be adjusted to provide efficient extraction. In some embodiments of the inventive concept the Ca content is controlled such that the mass ratio of Ca (in terms of CaO to water) can range from about 0.02 to about 0.5. In other embodiments the mass ratio of Ca can range from about 0.05 to about 0.25. In a preferred embodiment of the inventive process the mass ratio of Ca can range from about 0.1 to about 0.15.

The extraction process can be initiated as shown in 125 by the addition of an acid form of a counterion 130, in this instance hydrochloric acid (HCl), which generates an organic acid cation/counterion pair 140 (in this instance monoethanolamine hydrochloride (MEA+/Cl−)) to form a lixiviant solution. Monoethanolamine hydrochloride (MEA.HCl, HOC₂H₄NH₃Cl) conforms to Compound 1 as follows: one nitrogen atom (N₁) is bound to one carbon atom (R₁=C₂H₅O) and 3 hydrogen atoms (R₂, R₃ and H), and there is one chloride counteranion (X₁=Cl−). The extraction process can be performed at any temperature suitable to support solvation of the alkaline earth salt formed by reaction with the organic amine cation/counterion pair. In some embodiments of the inventive concept the extraction can be performed in a temperature range of about 0° C. to about 120° C. In other embodiments of the inventive concept the extraction can be performed within a temperature range of about 20° C. to about 100° C. In a preferred embodiment of the inventive concept the extraction can be performed within a temperature range of about 20° C. and about 70° C., advantageously reducing the need for temperature control equipment.

As shown in 145 the lixiviant can enter or mix with the mud/slurry and, as shown in FIG. 1, effectively extract an alkaline earth hydroxide, for example calcium hydroxide (Ca(OH)₂), by the formation of a soluble alkaline earth cation/counterion pair 150 (in this instance, calcium chloride (Ca(Cl)₂)). The reaction can be stirred during the extraction process in order to improve reaction kinetics. In some embodiments stirrer speeds can range from about 100 rpm to about 2000 rpm; in other embodiments of the inventive concept stirrer speeds can range from about 200 rpm to about 500 rpm. Equation 6 depicts a critical chemical reaction in such an extraction (in this case calcium, from steel slag that contains contaminants). Note that MEA.HCl dissociates in water into monoethanolammonium cation (HOC₂H₄NH₃+ (MEAH+)) and chloride anion (Cl−). Reaction products include soluble CaCl₂ and uncharged monoethanolamine (MEA)). Ca(OH)₂(s)+2HOC₂H₄NH₃+(aq)+2Cl−(aq)→CaCl₂(aq)+2HOC₂H₄NH₂(aq)+2H₂O(l)  Equation 6

The extraction process can be performed for any suitable length of time, as defined by the amount and quality of the material to be processed. In some embodiments of the inventive concept the extraction can be performed for 0.5 hours to 24 hours. In other embodiments the extraction can be performed for about 30 minutes. In preferred embodiments of the inventive concept the extraction can be performed for about 15 minutes. Depending in part on the organic amine species used in the lixiviant, the pH of the solution can change during the extraction process, for example increasing as the alkaline earth element is extracted from the sample. In some embodiments of the inventive concept the pH of the solution at the beginning of the extraction can range from about 6 to about 13. In other embodiments of the inventive concept the pH at the end of the extraction step can range from about 10 to about 12.

Extraction of a sample with a lixiviant leaves insoluble materials that are not desirable in the final product. These can be removed by a variety of means, including settling, centrifugation, and filtration, as in 165 of step 155. In preferred embodiments of the inventive concept insoluble materials are removed by filtration, for example in a filter press that produces a filter cake. In order enhance the efficiency of the process, a filter cake from such a filtration can be washed to remove additional extracted calcium. In some embodiments the filter cake can be treated with a wash volume that is about 10 times that of the wetness of the filter cake. In preferred embodiments of the inventive process lower volumes can be used, for example about 5 times that of the wetness of the filter cake or about 3 times that of the wetness of the filter cake.

Following separation of the soluble fraction or supernatant from the unreacted contaminants 165, the solubilized alkaline earth element can be recovered by the addition of a precipitant 170, for example carbon dioxide (CO₂), as shown in 175. The precipitant acts to form an insoluble salt with the alkaline earth element. Suitable precipitants include, but are not limited to, carbon dioxide, carbonate, bicarbonate, sulfate, and phosphate, and can be supplied as a gas (e.g. CO₂), salt, or acid. Accordingly, it should be understood that, while carbonate salts of alkaline earth elements are widely used as examples throughout this document, corresponding sulfate and/or phosphates salts are considered to be included in the inventive concept.

Surprisingly, inventors have found that CO₂ precipitation of alkaline earth chlorides (for example, CaCl₂) can proceed efficiently at an acidic pH (i.e., pH<7). Addition of CO₂ also generates the organic amine cation/counterion pair, as shown in 190 and in Equation 7, thereby regenerating the lixiviant. CaCl₂(aq)+2HOC₂H₄NH₂(aq)+2H₂O(l)+CO₂→CaCO₃(solid)+HOC₂H₄NH₃+(aq)+Cl−  Equation 7 In the exemplary reaction the precipitant forms calcium carbonate (CaCO₃) 180 which, being relatively insoluble, can be easily recovered for additional processing and, if desired, recovery of calcium. For example, CaCO₃ can be recovered using a filter press, as described above. The regenerated lixiviant can be recycled into the process 185, advantageously reducing the overall need for lixiviant and increasing process efficiency as more raw materials containing alkaline earths are processed.

The precipitation reaction can be performed at any temperature suitable to support the solubility of the precipitating agent (for example, CO₂) and maintain the insolubility of the precipitated salt. In some embodiments of the inventive concept the precipitation reaction can be performed at about 4° C. to about 100° C. In other embodiments the precipitation reaction can be performed at about 20° C. to about 80° C. In preferred embodiments of the inventive concept the precipitation can be performed at about 40° C. to about 80° C. The concentration of CO₂ gas supplied can range from about 0.1% to about 100%. In some embodiments of the inventive concept the concentration of CO₂ gas can range from 10% to about 100%. This advantageously permits relatively low quality sources of CO₂, for example flue gas or other waste gases, to be utilized. The CO₂-containing gas can be applied at any rate suitable for conversion of essentially all of the calcium present to CaCO₃ within a suitable time, for example about 3 hours to about 4 hours. Suitable flow rates can range from 1 L/hour/mol Ca to about 100 L/hr/mol Ca. In preferred embodiments of the inventive concept the flow rate for CO₂ containing gas can be about 10 L/hour/mol Ca to about 20 L/hour/mol Ca. The pH of the solution can change during the precipitation reaction.

The pH of a working solution can change during the precipitation step. In some embodiments of the inventive concept, the starting pH of the solution can range from about 9 to about 12, and can range from about 6 to about 8 at the end of the precipitation. Advantageously, this pH shift can be monitored to provide an indication of the progress of a precipitation reaction. Surprisingly, inventors have found that such a CO₂ precipitation of alkaline earth chlorides (for example, CaCl₂) in this process can proceed efficiently at an acidic pH (i.e., pH<7). The precipitation reaction can be performed until a suitable endpoint is reached. For example, in some embodiments the precipitation can be performed until the pH of the reaction remains below a specified setpoint (for example, a pH of about 8) for at least about 15 minutes.

Separation of the precipitate can be accomplished by any suitable method, including removing the soluble fraction from the tank (for example, by decanting, pumping, or siphoning), filtration, centrifugation or other application of centripetal force, or a combination of these. In some embodiments the precipitate is removed using a filter press. The resulting filter cake can be easily recovered for additional processing and, if desired, recovery of calcium. The regenerated lixiviant can be recycled into the next iteration of the process 185, advantageously reducing the overall need for lixiviant and increasing process efficiency as more raw materials containing alkaline earths are processed.

It should be noted that the choice of lixiviant can allow for the selective extraction of calcium in this example because it does not react with other metals (ME) or metal oxides/hydroxides (MEO_(x)) in the alkaline earth source material, as shown in Equation 8 and Equation 9. ME(s)+HOC₂H₄NH₃+(aq)→NO REACTION  Equation 8 MEO_(x)(s)+HOC₂H₄NH₃+(aq)→NO REACTION  Equation 9

The soluble calcium salt and the soluble MEA from Equation 6 can easily be separated from the insoluble solid residue. Once separated, the soluble aqueous fraction can used as-is if the target process can withstand the small quantity of lixiviant as a contaminant, or the solution can be furthered processed as needed.

In an alternative embodiment of the inventive concept, a solution containing an alkaline earth cation/counterion complex as shown in Equation 6 can be concentrated or diluted to a desired strength as required by the end user. Alternatively, such a solution can be boiled down or evaporated completely, leaving an alkaline earth element cation/counterion salt and/or various hydrates thereof, depending on how vigorously the mixture is dried. The residual uncharged organic amine could also be removed by this process and optionally captured for reuse. The dried alkaline earth element chlorides could be further processed into oxides via thermal oxidation, precipitation with agents such oxalic acid, sodium hydroxide, potassium hydroxide or other precipitating agents.

There are of course many possible lixiviants of the form of Compound 1, and there are likewise many alkaline earth element sources. While the examples provided have described the action of two organic amine lixiviants (i.e., ammonium chloride and monoethanolamine hydrochloride (a.k.a. monoethanolammonium chloride) with one particular source (steel slag) of a particular alkaline earth element (calcium) other examples of process of the inventive concept can utilize organic amine cation/counterion pairs such as ammonium acetate, monoethanolammonium acetate, ammonium nitrate, or monoethanolammonium nitrate. Alternatively, biologically derived lixiviants such as the amino acid glycine (or a salt of itself) or the hydrobromide salt of poly-L-lysine can be used. Similarly, while examples note the use of steel slag, other sources (such as calcite, dolomite, gypsum, plagioclases, amphiboles, pyroxenes, and garnets) are suitable. Alternatively, systems, methods, and compositions of the inventive concept can be utilized to recover alkaline earth elements from agricultural waste, consumer waste, industrial waste, scrap or other excess materials from manufacturing processes, or other post-utilization sources.

Many alkaline earth elements can form hydroxides; most of these have very limited solubility in water. These hydroxides also have varying degrees of basicity. While calcium hydroxide as produced from various mineral sources has been cited as an example there are many other alkaline earth elements that form suitable bases in water. Examples of other elements that, in hydroxide form, are suitable for use in systems and methods of the inventive concept include beryllium, magnesium, strontium, barium, and radium. Such salts have different basicities, which can be paired with organic amine based lixiviants of different acidities to provide selective recovery.

It should also be noted that systems, methods, and compositions of the inventive concept are not limited to one alkaline earth species being extracted with one particular lixiviant or set of anions. Multiple alkaline earth species with various organic amine based lixiviants and various anions (or acids) can be used in sequence or in parallel to extract a particular mixture of metals or to produce a particular mixture of metal salts.

As described above, lixiviants of the inventive concept can be applied in a variety of methods. Examples of some of these methods are depicted schematically in FIG. 2, FIG. 3, and FIG. 4.

FIG. 2 depicts a method of the inventive process 200 in which a sample 210, for example an ore, mineral, or other source of an alkaline earth element, is mixed with a lixiviant 220. The lixiviant can include one or more organic amine species as described above in the form of a cation, coupled with a suitable counterion. Suitable counterions can include halides. In a preferred embodiment of the inventive concept the counterion is chloride (Cl−).

A sample 210 can be a calcium-containing ore (for example dolomite or gypsum), a byproduct of a manufacturing process (for example, steel slag), or any suitable calcium source. The sample 210 can be treated prior to mixing with the lixiviant 220. For example, the components of the sample 210 can be reduced in size, for example through milling, grinding, pulverizing, or sifting. Such processes improve the surface area to volume ratio of elements of the sample and can serve to increase reaction rates. In some embodiments a sample can be chemically treated, for example through exposure to strong bases (such as sodium hydroxide) or oxidized through exposure to air at elevated temperatures. Such chemical treatments can serve to generate alkaline earth metal salts (for example, hydroxides or oxides) and to alter the physical structure of the sample or components of the sample.

On interacting with the lixiviant 220, alkaline earth elements in the sample interact with organic amine cations and counterions to form a soluble alkaline earth element cation/counterion complex that is solubilized in the supernatant 230, along with an uncharged organic amine. The pH of this portion of the reaction process can be alkaline, i.e., ranging from about 7.5 to about 14. In some embodiments of the inventive concept the pH can range from about 10 to about 12. Unwanted contaminants are not solvated, and remain behind as insoluble material, for example as a treated sample 240 that can be further processed if desired.

The supernatant 250 can be separated from the insoluble materials of the treated sample 240 by a variety of processes, including settling, filtration, or centrifugation, either alone or in combination. The alkaline earth cation 260 can be recovered from the supernatant 250 by any suitable means, including electrodeposition, precipitation, and ion exchange. In a preferred embodiment of the inventive concept the alkaline earth cation is recovered by the addition of a precipitant (Pr) to produce an insoluble alkaline earth salt that is easily recovered. Such precipitants can be an H+ donating species suitable for forming insoluble salts of alkaline earth elements while regenerating an organic amine cation, for example CO₂ or carbonic acid, chromic acid, or sulfuric acid. In a preferred embodiment of the inventive concept the precipitant (Pr) is CO₂ or carbonic acid. Surprisingly, inventors have found that this precipitation can be performed at a pH of less than 7. In such an embodiment a precipitation step can be performed at a pH between about 6 and about 7. In a preferred embodiment a precipitation step can be performed at a pH of about 6.7. The uncharged organic amine remaining in the supernatant 250 can, in turn, be regenerated 270 in this process to form an organic amine cation that can form part of a lixiviant 220 that can be used in the next iteration of the reaction. This recycling of the lixiviant greatly reduces consumption through multiple cycles of the process and advantageously reduces environmental impact and expense.

Other embodiments of the inventive concept can advantageously utilize the selective complex formation and solubility of components of methods of the inventive concept to recover different alkaline earth elements from the same sample. One example of such a method is shown in FIG. 3. As shown, such a method can be a chain of reactions that are, essentially, one or more repetitions of the method shown in FIG. 1 applied to a progressively depleted sample. In an example of such a method 300, a sample 305 and a first lixiviant 310 are brought into contact with each other. The first lixiviant 310 includes a first organic amine cation and a counterion, and reaction 315 with the sample 305 produces a first depleted sample 320 and a first supernatant 325 that includes a first alkaline earth cation, a counterion, and an uncharged organic amine. The first depleted sample 320 includes materials that were not reactive with the first lixiviant, which can include additional alkaline earth elements, other valuable materials, and unwanted contaminants. It can be separated from the first supernatant 325 by any suitable method, including settling, filtration, and centrifugation, either alone or in combination. The first alkaline earth cation can be recovered from the first supernatant 325 by any suitable means, including electrodeposition, precipitation, and ion exchange. In a preferred embodiment of the inventive concept a first precipitant (Pr1) can used that generates an insoluble first alkaline earth salt and regenerates the first organic amine cation/counterion pair 330. In such an embodiment the uncharged first organic amine remaining in the supernatant 325 can, in turn, be regenerated 360 to give a first organic amine cation that can form part of a first lixiviant 310 that can be used in the next iteration of the process.

The first depleted sample 320 can, in turn, be contacted 340 with a second lixiviant 335 that includes a second organic amine cation/counterion pair. Reaction with the first depleted sample 320 produces a second depleted sample 350 and a second supernatant 345 that includes a soluble second alkaline earth element cation/counterion complex and uncharged second organic amine. The second alkaline earth cation can be recovered from the second supernatant 345 by any suitable means, including precipitation, electrodeposition, and/or ion exchange. In a preferred embodiment of the inventive concept a second precipitant (Pr2) can used that generates an insoluble second alkaline earth salt and regenerates the second organic amine cation/counterion pair 355.

Such precipitants can be an H+ donating species suitable for forming insoluble salts of alkaline earth elements while regenerating an organic amine cation, for example CO₂ or carbonic acid, chromic acid, or sulfuric acid. The regenerated second organic amine/counterion pair can in turn be recycled 365 for use in the next iteration of the process. In some embodiments of the inventive concept the first precipitant and the second precipitant are the same species. In other embodiments of the inventive concept the first precipitant and the second precipitant are different species. In a preferred embodiment of the inventive concept the first precipitant and the second precipitant are CO₂ or carbonic acid. In some embodiments of the inventive concept the second depleted sample 350 is subjected to further rounds of treatment with lixiviants in order to recover additional valuable materials. This recycling of the lixiviants advantageously reduces the overall amount of organic amines used as the process is repeated, which limits both the environmental impact of such operations and permits considerable savings in materials.

Another embodiment of the inventive concept that permits recovery of two or more alkaline earth elements from a sample is shown in FIG. 4. In such a method 400 a sample 410 is contacted with a lixiviant 420 that includes a first organic amine cation/counterion pair and a second organic amine cation/counterion pair. This mixture 430 results in a treated sample 450 and a first supernatant 440. This first supernatant can include a first alkaline earth element cation/counterion pair, a second alkaline earth element cation/counterion pair, a first uncharged organic amine, and a second uncharged organic amine. The first alkaline earth cation 460 can be recovered from the first supernatant 440 by any suitably selective means, including precipitation, electroplating, or ion exchange. In a preferred embodiment of the inventive concept, the first alkaline earth element can be recovered by adding a first precipitant (Pr1) that selectively forms an insoluble salt of the first alkaline earth element (or cation) 460. For example, in a sample containing a mixture of magnesium and calcium, the calcium can be recovered in this step of the reaction by the addition of chromic acid as a first precipitant (P1) to form relatively insoluble calcium chromate (CaCrO₄); relatively soluble magnesium chromate (MgCrO₄) would remain in solution.

Recovery of the second alkaline earth cation from the second supernatant 470 also yields a regenerated lixiviant. The second alkaline earth cation can be recovered from the second supernatant 470 by any suitable means, such as precipitation, electrodeposition, or ion exchange. In some embodiments of the inventive concept, the second alkaline earth element can be recovered by adding a second precipitant (Pr2) that forms an insoluble salt of the second alkaline earth element and completes regeneration of the lixiviant 480. For example, in a sample containing a mixture of magnesium and calcium, the magnesium can be recovered in this step of the reaction from a supernatant resulting from chromic acid treatment by the addition of CO₂ or carbonic acid as a second precipitant (P2) to form relatively insoluble calcium carbonate (CaCO₃). The regenerated lixiviant can in turn be recycled 490 in the next iteration of the process.

In some embodiments of the inventive concept the first organic amine and the second organic amine (and their respective cations) can be different molecular species with different acidities and/or specificities for alkaline earth elements. In other embodiments of the inventive concept the first organic amine and the second organic amine can be the same molecular species, with selectivity between the first alkaline earth element and the second alkaline earth element being provided by the method used for their recovery from supernatants. For example, utilization of different precipitating species, utilization of the same precipitating species under different conditions (for example, concentration, temperature, pH, or a combination of these), utilization of ion exchange media with different selectivities, or combinations of these approaches can be used to provide selective recovery of the alkaline earth elements of a sample. It should be appreciated that, as described in the processes illustrated in FIG. 2 and FIG. 3, that regeneration and re-use of the lixiviant through repeated iterations advantageously reduces the amount of organic amine needed, which limits both the environmental impact of such operations and permits considerable savings in materials.

A specific example of the recovery of calcium from steel slag is shown in FIG. 5 and FIG. 6. FIG. 5 shows the composition of a typical steel slag, showing a complex mixture of various metal oxides including calcium oxide (CaO), which becomes calcium hydroxide (Ca(OH)₂) on exposure to water. Processing of such a steel slag is shown diagrammatically in FIG. 6. Initially, steel slag (or an alternative calcium source) is ground 600 to less than around 125 μm. This greatly increases the surface area available for reaction. Water and lixiviant are mixed 610 in a suitable ratio, which can range from 1% to about 50%. The ground slag and aqueous lixiviant are mixed 620 and stirred or agitated for a time sufficient to form the calcium cation/counterion pair, in this instance approximately 10 minutes. The solid residue, which is depleted of calcium, is removed by filtration 630 and the liquid fraction or supernatant is processed by adding carbon dioxide (or an equivalent, such as carbonic acid) to precipitate calcium carbonate (CaCO₃) 640. This process also regenerates the lixiviant. The CaCO₃ can then be prepared for further processing by washing, dilution into a slurry, and so on 660, while the regenerated lixiviant is recycled for re-use in the next iteration of the process 670.

Examples of the recovery of calcium by systems, methods, and compositions of the inventive concept are shown in FIG. 7A-FIG. 7D. FIG. 7A shows the change in pH over time as calcium is extracted from low-grade lime using monoethanolamine-HCl (MEACL) as the organic amine lixiviant. In this reaction 5 grams of low-grade lime was mixed with 50 grams of water containing the lixivant at a lixiviant to calcium molar ratio of 2.1:1, while stirring 400 rpm. The reaction was allowed to proceed for 23 minutes. FIG. 7B shows the results of a similar study, in which the pH was monitored over time as calcium is extracted from low-grade lime using glycine as the organic amine lixiviant. It should be appreciated that as an amino acid glycine can be advantageously derived from biological sources and that, due to its zwitterionic nature, glycine can act as its own counterion. In this reaction 5 grams of low-grade lime was mixed with 50 grams of water containing the lixiviant at a lixiviant to calcium molar ratio of 2.1:1, while stirring at 400 rpm. The reaction time was allowed to proceed for 24 minutes. FIG. 7C shows the results of recovery of extracted calcium using a precipitant, in this instance CO₂. In this example pH was monitored as CO₂ was perfused through calcium extracted from low grade lime using monoethanolamine-HCl as the lixiviant. The reaction was performed for 11 minutes as 100% CO₂ was perfused through the solution at 20 mL per minute at a temperature of 22° C., while stirring at 400 rpm.

A photomicrograph of an exemplary product from the extraction of calcium using systems, methods, and compositions of the inventive concept is shown in FIG. 7D. The reaction was performed using 10 grams low-grade (˜50% CaO content) lime, which was treated with 19.7 grams of monoethanolamine-HCl in 100 grams water and stirred at 400 rpm for 30 minutes. Solid residue was removed by filtration and the filtrate perfused with 100% CO₂ at a flow rate of 20 mL/min at a temperature of 60° C., until the pH was less than 8 for 15 minutes. The yield of precipitated calcium carbonate (PCC) was 86%.

As shown in FIG. 8A-8B, the processes of the inventive concept can advantageously be readily adapted to the infrastructure of current processing plants. FIG. 8A shows a schematic of typical components and material transfers of a commercial powdered calcium carbonate (PCC) plant. Limestone is received in a limestone bin and calcined in a kiln to produce calcium oxide. Flue gases from the kiln are processed in a gas filter and scrubber. Calcium oxide from the kiln is transferred to a quicklime bin, and then to a slaker where it is mixed with water to produce calcium hydroxide. Calcium hydroxide from the slaker is transferred to a cooling unit to remove heat from this exothermic process, and subsequently transferred to one or more reactors. The reactors receive CO₂, part of which can be recovered from flue gas by the scrubber, and mix it with the calcium hydroxide suspension. CO₂ forms carbonic acid in the water of the suspension and slowly reacts with the solids of the calcium hydroxide suspension to produce calcium carbonate. The solid calcium carbonate is transferred to a postscreen tank and then a postscreen tank, and finally to a filter press in order to remove unreacted material and a portion of the water. Material recovered by the filter press is transferred to a dryer for thorough removal of water to generate the powdered calcium carbonate product.

The arrangement and nature of the components in such a conventional plant allows adaptation to lixiviant-based methods with minimal disruption, as shown in FIG. 8B. As shown, new components and material transfer paths are shown with dotted lines. In the modified plant, the slaker receives a water/lixiviant mixture 800, which reacts with the calcium hydroxide formed in the slaker to form a soluble calcium ion/counterion complex that can be transferred to a cooling unit and subsequently to a filterpress 810 that separates unreacted material 820 from the calcium-containing solution, which is transferred 830 to one or more reactors. Reaction with carbon dioxide, part of which can be obtained from the flue gas scrubber associated with the kiln, rapidly generates calcium carbonate in a solution-phase reaction, in the process regenerating the lixiviant. This regenerated lixiviant can be retrieved from the post screen tank and returned to the slaker for the next iteration of the process. FIG. 8C shows how a multi (for example, 3 to 5) effect evaporators 840 can be added to the effluent lines 850 from the filter presses of a lixiviant-adapted plant to concentrate regenerated lixiviant for re-use and to recover solvent (for example, water) 860 for use in washing the calcium carbonate produced by the process, in order to adjust and optimize the lixiviant concentration while reducing material costs.

Advantageously, efficiencies of plants operated in such a manner can be very high, as shown in FIG. 8D-8E. FIG. 8D shows mass balances for a typical iteration of a calcium isolation from steel slag. Only 7.22 kg of lixiviant is lost for 4,301 kg of 97.8% calcium carbonate produced. Over 99.9% of the lixiviant used in such a reaction is recycled from previous iterations. Similarly, FIG. 8E shows mass balances for a typical iteration of a calcium isolation from lime. Only 9.39 kg of lixiviant are lost for 4,301 kg of 99.8% pure calcium carbonate produced. Over 99.85% of the lixiviant used in such a reaction is recycled from previous iterations.

In exemplary processes described above the extraction of alkaline earth salts from raw materials (leaving insoluble particulate waste) and the precipitation of alkaline earth carbonates from the resulting solution have been separate, segregated processes. This approach can enable production of a pure calcium carbonate precipitate (PCC) to be used in industries such as papermaking. However, in some embodiments of the inventive subject matter extraction and precipitation can occur simultaneously within a single container or enclosure as shown schematically in FIG. 9a and FIG. 9B. FIG. 9A depicts such a process using a lixiviant. As shown, in such a process 900A, a raw material containing an extractable alkaline earth salt (i.e. raw material) 910 (for example, slag from steel processing, dolite, ash from municipal waste, etc.), a lixiviant 920, and a source of carbon dioxide 930 (for example, a carbon dioxide containing gas) are supplied to a vessel or other enclosure that serves as a reactor 940. In a preferred embodiment of the inventive concept the lixiviant 920 is supplied at substoichiometric amounts relative to the extractable alkaline earth salt content of the raw material 910. It should be appreciated that only a portion of the alkaline earth content of a raw material 910 can be present in the form of salts or other compounds that are subject or accessible to extraction. The reactor 940 can also include provisions for the venting of excess or unreacted gas 950 (for example, non-CO₂ gas components originating from the source of carbon dioxide 930). Within the reactor 940 the reaction products (i.e precipitated alkaline earth carbonate and regenerated lixiviant) along with unreacted and insoluble waste produce a mixture that is supplied to a separator 960. The separator 960 can segregate the reaction products produced in the reactor 940 and separate them from the waste or unreacted materials by any suitable means, including the application of centripetal force, magnetic attraction, settling by gravity, and/or filtration. Segregated outputs from the separator 960 include an alkaline earth carbonate 970, unreacted materials 980 derived from the raw material 910, and regenerated lixiviant 990. In some embodiments of the inventive concept, regenerated lixiviant 990 can be returned to the reactor 940 for even higher efficiency. It should also be appreciated that the unreacted waste material, having been depleted of one or more alkaline earth elements, can be a commercially valuable material (having been, essentially, enriched in the remaining unreacted materials) and may be collected for further processing rather than disposed of.

FIG. 9B depicts an alternative embodiment of such a method, specifically one that does include an added lixiviant. As shown, in such a process 900B, a raw material containing an extractable alkaline earth salt (i.e. raw material) 910 (for example, slag from steel processing, dolite, ash from municipal waste, etc.) and a source of carbon dioxide 930 (for example, a carbon dioxide containing gas) are supplied to a vessel or other enclosure that serves as a reactor 940. It should be appreciated that only a portion of the alkaline earth content of a raw material 910 can be present in the form of salts or other compounds that are subject or accessible to extraction. The reactor 940 can also include provisions for the venting of excess or unreacted gas 950, which can, optionally, be directed back to the reactor 940 to serve as at least part of the carbon dioxide source 930. Within the reactor 940 the reaction products (i.e precipitated alkaline earth carbonate and regenerated lixiviant) along with unreacted and insoluble waste produce a mixture that is supplied to a separator 960. The separator 960 can segregate the reaction products produced in the reactor 940 and separate them from the waste or unreacted materials by any suitable means, including the application of centripetal force, magnetic attraction, settling by gravity, and/or filtration. Segregated outputs from the separator 960 include an alkaline earth carbonate 970, unreacted materials 980 derived from the raw material 910, and regenerated lixiviant 990. It should also be appreciated that the unreacted waste material, having been depleted of one or more alkaline earth elements, can be a commercially valuable material (having been, essentially, enriched in the remaining unreacted materials) and may be collected for further processing rather than disposed of.

Other embodiments of the inventive concept can incorporate multiple separators, as shown schematically in FIGS. 10A and 10B. FIG. 10A depicts such a process using a lixiviant. In such a process 1000A raw material containing an extractable alkaline earth salt 1010, and a source of carbon dioxide 1020, and a lixiviant 1030 are supplied to a reactor 1040. In some embodiments of the inventive concept, raw material can be mixed with a lixiviant or lixiviant containing solution and supplied to the reactor 1040 as a mixture or slurry. Within the reactor 1040 the reactions in which the lixiviant generates soluble species from accessible alkaline earth salts of the raw material, formation of insoluble alkaline earth carbonates, and regeneration of the lixiviant species take place. After a suitable period of time the precipitated alkaline earth carbonate, regenerated lixiviant, and particulate unreacted materials are transferred to a separator 1060, or optionally a separator 1060 and a secondary separator 1062 or a plurality of secondary separators 1062, 1064. Such secondary separators can be similar or identical to the separator 1060, or can differ in capacity, operating principle, or selectivity from the separator 1060. In some embodiments, secondary separators 1062 and 1064 can differ from each other. Separation produces a segregation of the alkaline earth carbonate 1070 from unreacted or waste materials 1080, and generates a stream of regenerated lixiviant 1090 that is returned to the reactor 1040. Such a process obviates (at least partially) the need to add new lixiviant to each performance of the process 1000 and advantageously results in considerable savings in time, material, and waste treatment costs.

FIG. 10B depicts an alternative process carried out in the absence of added lixiviant. In such a process 1000B raw material containing an extractable alkaline earth salt 1010 and a source of carbon dioxide 1020 are supplied to a reactor 1040. Within the reactor 1040 the reactions in which the lixiviant generates soluble species from accessible alkaline earth salts of the raw material, formation of insoluble alkaline earth carbonates, and regeneration of the lixiviant species take place. After a suitable period of time the precipitated alkaline earth carbonate, regenerated lixiviant, and particulate unreacted materials are transferred to a separator 1060, or optionally a separator 1060 and a secondary separator 1062 or a plurality of secondary separators 1062, 1064. Such secondary separators can be similar or identical to the separator 1060, or can differ in capacity, operating principle, or selectivity from the separator 1060. In some embodiments, secondary separators 1062 and 1064 can differ from each other. Separation produces a segregation of the alkaline earth carbonate 1070 from unreacted or waste materials 1080. In such embodiments unreacted carbon dioxide 1050 can be collected and, optionally, returned to reactor 1040 as at least part of the carbon dioxide source 1020.

Still another embodiment of a method of the inventive concept is shown in FIG. 11, where the solubilization and precipitation reactions are partially segregated from one another. In such a process 1100, raw material that includes extractable alkaline earth salts (i.e. raw material) 1110 is supplied to a primary reactor 1140. In some embodiments lixiviant or a lixiviant containing solution 1120 is also supplied to the primary reactor 1140. In still other embodiments, raw material and lixiviant can be mixed and the resulting mixture or slurry supplied to the primary reactor 1140. Following extraction of the alkaline earth from the raw material 1110, the reaction mixture is transferred to a primary separator 1160, which separates unreacted solids from alkaline earth-bearing solution 1166. The alkaline earth-bearing solution 1166 is transferred to a secondary reactor 1167, where a source of carbon dioxide 1130 (for example, a gas that contains carbon dioxide) is added. This results in the formation of an alkaline earth carbonate in the form of a precipitate. The resulting precipitate slurry 1168 is transferred to a secondary separator 1169. Secondary separator 1169 produces a liquid fraction that contains regenerated lixiviant, which can be in the form of a primary reactor stream 1190A and/or in the form of a primary separator stream 1190B. The carbonate solids 1170 segregated from the precipitate slurry 1168 by the secondary separator 1169 can be recovered for use or, alternatively, washed by the application of water or another suitable wash solution to the recovered solid (thereby producing a wash waste stream 1195) prior to recovery of the carbonate solid 370 from the secondary separator 1169.

Systems and methods of the inventive concept can efficiently extract alkaline earths from raw materials using only a fraction of the lixiviant utilized in prior art processes. In essence, the extraction-precipitation cycle is repeated continuously utilizing a small (for example, sub-stoichiometric) amount of lixiviant that is regenerated during the cycle. In this way, the amine-containing lixiviant species acts a pseudocatalyst. Surprisingly, the inventors have found that alkaline earths can also be extracted in the absence of any added lixiviant.

In exemplary single step processes 900A, 900B, 1000A, 1000B, an 1100A as described above, where alkaline earth carbonates are precipitated in the presence of the raw material and/or extracted raw materials (which are generally supplied as particulates), efficient recovery or segregation of the desired alkaline earth carbonate is dependent upon distinguishing between particulate species in the resulting mixture. In preferred embodiments of the inventive concept, reaction conditions and raw materials are selected so that the particle density, particle shape, particle size, effective particle hydrodynamic radius, and/or magnetic properties of the particles of alkaline earth carbonate generated by the process and the density, particle shape, effective particle hydrodynamic radius, and/or magnetic properties particles of raw material (and/or extracted raw material) are sufficiently distinct to permit effective separation (e.g. at least 50%, 60%, 70%, 80%, 90%, or greater than 90% segregation of the precipitated alkaline earth carbonate from the raw material or extracted raw material). For example, in some embodiments of the inventive concept process conditions are selected such that the particulates forming the alkaline earth carbonate precipitate can range from 0.1 μm to 10 μm in size. In processes that utilize settling or decantation for separation the product alkaline earth precipitate can have a settling velocity that is substantially lower (i.e. 50% or less) than that of the extracted raw material, which can range in size from 50 μm to 500 μm in diameter. For example, steel slags and extracted steel slags with a size range of 100 μm to 500 μm (with an average diameter of 230 μm) was found to settle approximately 130 times more quickly than a product calcium carbonate precipitate particle have an average particle diameter of 15 μm. It should be appreciated that, for some raw materials, the density of the raw material, the extracted raw material, and the product alkaline earth precipitate can be similar, in which case separation behavior can be largely determined by particle size, shape, and/or effective hydrodynamic radius. In a preferred embodiment, the diameter of an alkaline earth carbonate precipitate can range from 250 nm to 10 μm. The mean size and size distribution of such an alkaline earth carbonate product can be controlled, for example, by modulating the stirring speed within the reactor and/or the rate of CO₂ addition. A wide range of particle sizes is acceptable for the raw material, and the optimal particle size for a given process may be determined by the economic impact of milling or grinding of the raw material and yield (as larger particle sizes can be associated with reduced yield). In a preferred embodiment the average particle size of the raw material is 200 μm. Suitable operating temperatures for single-step processes are similar to those of the two-step processes described above. In a preferred embodiment of the inventive concept a single-step process is performed at approximately 60° C.

As noted in the processes described above, separation of the precipitated alkaline earth carbonates and the extracted raw material is performed in a separator. The separators referred to in the above described processes can utilize a wide variety of processes and/or physical phenomena to segregate solids from liquids. Similarly, suitable separators can perform segregation operations in a fixed volume or batch format or on a continuous basis, as fits the requirements of the process. For example, suitable separators can use simple fractionation methods such as gravitational settling, decanting, or desilting. Alternatively, suitable separators can perform filtration, for example using press filters, rotary pressure filters, and/or vacuum belt filters. In other embodiments, a separator can use centrifugal force, for example via a centrifuge or hydrocyclone. In still other embodiments a separator can utilize magnetic effects (for example as provided by a magnet or electromagnet) to separate magnetically responsive (i.e. magnetic, diamagnetic, and/or paramagnetic) materials, for example treated steel slag particulates, from other materials that do not respond to magnetic fields.

It should be appreciated that centrifugal separation techniques advantageously permit continuous separation, and can be configured to segregate micron-scale particles of carbonate precipitates from larger and/or denser raw material residue particles from a particulate mixture generated by simultaneous performance of the above described extraction and precipitation reactions. In a preferred embodiment of the inventive concept, an alkaline earth carbonate production process can be configured such that raw material and a source of carbon dioxide are supplied in a continuous fashion to a reactor supplied with a solution containing a lixiviant species, with a stream of liquid containing suspended particulates being directed to a separator that segregates alkaline earth carbonate from extracted raw material and returns a solution containing the lixiviant species to the reactor. Alternatively, the raw material, the source of carbon dioxide, or both can be supplied to the reactor in a pulsatile or intermittent fashion.

Another embodiment of the inventive concept is a system that is configured to perform the single-step methods described above. In some embodiments of the inventive concept, such a system can switch between continuous and intermittent modes of operation to accommodate the needs of the operation and/or the availability of materials. Such a system can include a reaction enclosure, in which raw material, a lixiviant solution, and a precipitant (such as CO₂ containing gas) are brought into contact with one another. Such a reaction enclosure can include one or more sensors that provide data related to the progress of the reaction. For example a reaction enclosure can include a device for characterizing the pH of the reaction mixture, which as shown in FIG. 16, FIG. 17A, and FIG. 17B changes during the course of the reaction. Other suitable sensors include an ion-selective electrode, densitometer, spectrophotometer, nephelometer, and particle characterization devices based on the Coulter principle. Such sensors can provide data to a controller, which in turn can modulate the rate of the reaction by controlling the rate of introduction of raw material, lixiviant species, and/or precipitant to the reaction enclosure. The reaction enclosure is in communication with a separator. In some embodiments of the inventive concept such a reaction enclosure (or a portion thereof) can act as a separator. For example, a reaction enclosure can be configured or have a portion that is configured for use in decanting (for example, being configured as a vertically oriented cylinder or cone). In other embodiments a separating device can lie within or be in fluid communication with the reaction enclosure. For example, a reaction enclosure can be in fluid communication with a filter device (such as a filter press) or a centrifugal separator (such as a centrifuge or a hydrocyclone). Such separating devices permit separation of the precipitated alkaline earth salt from the extracted and/or unreacted raw material, and from the liquid phase of the reaction mixture.

A system of the inventive concept can be configured to operate in a discontinuous or in a continuous manner. When operated in a discontinuous manner an amount of raw material, an amount of lixiviant species, and an amount precipitant are provided to the reaction enclosure and the reactions described above allowed to proceed. At least one of these reactants (for example the raw material and/or the lixiviant species) is added to the reaction enclosure as a single bolus. This results in the formation of an alkaline earth containing precipitant that is separable from the extracted raw material and a solution phase that includes the lixiviant species. At the completion of the reaction the reaction products are separated, and the process repeated in a repetition of the reaction cycle. When operated in a continuous manner raw material, lixiviant, and precipitant (for example, a CO₂ containing gas) are added to the reaction enclosure essentially continuously (i.e. continuously or as a continuous series of small, distinct volumes), generating an equilibrium reaction mixture that is separated essentially continuously (i.e. continuously or as continuous series of small, distinct volumes). It should be appreciated that in such an embodiment the addition of lixiviant species is inclusive of the return of regenerated lixiviant resulting from the separation process to the reaction enclosure.

The inventors have found that such single-step processes (in which extraction of the alkaline earth using a lixiviant is coupled with precipitation of the alkaline earth and regeneration of the lixiviant species) are more efficient for extraction of alkaline earths, for example calcium, from raw materials than traditional two-step or segregated processes (in which extraction of the alkaline earth using a lixiviant is decoupled from precipitation of the alkaline earth and regeneration of the lixiviant species). Surprisingly, the inventors have also found that such single step processes can be performed in the absence of an added lixiviant and still efficiently extract an alkaline earth (for example, calcium) from a raw material. This advantageously both simplifies the process and eliminates exposure to lixiviant compounds. Examples that illustrate this follow.

EXAMPLES

Raw Materials and Methods

Two basic oxygen furnace (BOF) slags were utilized as raw material sources of extractable calcium oxides/hydroxides in these experiments. One sample consisted of raw fines from U.S. Steel Lake Erie Works, Canada, while the other was a sample from a reject stream generated by a slag recycling plant at Ruukki Metals Raahe Works, Finland. The compositions of these slags, analyzed with XRF, are presented in FIG. 19. Because the materials had been stored outdoors, they were calcined for three hours at 900° C. before extraction to reduce any material that may have already been carbonated by environmental factors.

The U.S. Steel slag was directly sieved to the desired size fractions, while Ruukki slag was first milled to smaller particle sizes and then sieved to the final desired size fractions. In the experiments a specified amount of slag was mixed with a volume of ammonium chloride lixiviant solution of known concentration in a covered beaker with a magnetic stirrer. Solution pH was recorded using an Omega PHH-SD1 pH meter.

Two types of process tests were conducted; two-step processes were performed as described in the prior art, and served to provide estimates of the extractable calcium available through traditional processes. Towards this end the ammonium chloride lixiviant solution was provided in molar excess (see FIG. 20) to avoid stoichiometric limitations. The raw material was first mixed with the NH₄Cl solution for 30 minutes. After removal of the residual extracted raw material by filtration, carbon dioxide gas was bubbled through the filtrate for 45 minutes. The obtained carbonates were filtered from the solution, and the amount of extractable calcium was determined by gravimetric analysis following drying the solids overnight at 80° C.

In one-step processes of the inventive concept, the raw material was first suspended in water and the pH allowed to stabilize. Following this, sub-stoichiometric quantities of ammonium chloride salt (as determined from the results of the amount of extractable calcium recovered from single step processes above) were added to the suspension and carbon dioxide gas was fed to the reactor until solution pH decreased to 8.00, after which the flow of CO₂ was stopped for 30 minutes, allowing pH to increase and stabilize. To insure complete precipitation of the calcium, the flow CO₂ gas was re-started while monitoring the pH decrease. The flow of CO₂ gas was continued for 10 minutes after the pH reached 8.00. The experiment was finished without gas flow, recording the pH increase for seven additional minutes. These time periods were selected based on the single step process described above to permit comparison between the different experiments, and can be further optimized. The components of the final reaction mixture were separated by decanting followed by filtration. The lighter calcium-rich fraction was first decanted from the reaction vessel to a filter, while the heavier slag residue particles remained at the bottom of the vessel and were filtered separately. Before weighing, the samples were dried overnight at 80° C.

In mass balance calculations it was assumed that all the observed mass increase resulted from captured carbon dioxide. The produced calcium carbonate amount was calculated based on this assumption. The obtained fractions were analyzed with SEM/EDX to characterize their composition and particle structure. Select liquid samples were analyzed with ICP-OES.

The experimental conditions from this series are listed in FIG. 20. The slag-to-liquid ratio of the suspension was 100 g/L, temperature and pressure were ambient, and a pure CO₂ gas flow rate of 75 mL/min was used.

Results

Calcium extraction and carbonate precipitation from different slag particle size fractions (experiments 1-16) were studied with both one- and two-step methods. FIG. 12A shows that for the largest slag particles (500-1000 μm), the yields of the compared process alternatives were equivalent. While yield increased as the raw material particle size decreased for both one-step and two-step processes, the increase in yield for the single-step process was dramatically greater than that of the two-step process for both raw materials. Surprisingly, as shown in FIG. 12B, in a one-step process with small <53 μm slag particles the carbonate yield almost tripled compared to the prior art two-step process (i.e. showed an almost 200% increase over and above the yield of the two-step process). With intermediate particle sizes the increase in carbonate production was 100-150%. Both U.S. Steel slag and Ruukki slag followed the same trend, suggesting a critical particle size, above which both processes result in similar carbonate yields.

Interestingly, U.S. Steel slag with particle sizes 53-250 μm displayed a plateauing in carbonate yield. The yield increased once particle sizes were reduced to below 53 μm. It is known that during slag formation calcium tends to enrich in small particles. Thus, the U.S. Steel slag size fractions may have differences in composition, with the sieving process resulting in a segregation of these different particle compositions. Since the Ruukki slag sample was milled from large particles prior to sieving into particle size fractions, the different fractions of the Ruuki slag may have a consistent composition and more directly show the effect of particle size.

FIG. 13 shows the masses of the separated fractions produced by one-step processes applied to U.S. Steel slag (i.e. experiments 6-10). The mass of the lower density “carbonate mixture” fraction increases with decreasing slag particle size. This is not only because of the increased recovery of calcium carbonate, but also because the smaller slag residue particles do not separate as efficiently from the carbonate particles by decanting methods. This is especially true for the <53 μm slag particles; the mass of “residue” (i.e. extracted raw material) was decreased, indicating that the extracted slag particles may have similar settling properties to those of the produced calcium carbonate.

In experiments 1-19 the molar ratio of ammonium chloride and reactive calcium oxide varied in part due to changes in the amount of extractable calcium related to particle size effects. Earlier research has shown that in the two-step process ammonium salt solution concentration does not significantly affect the calcium yield, assuming that the chosen concentration exceeds the stoichiometric limits defined by Scheme 1. However, it is known that at ammonium salt concentrations above 1.5 M the selectivity towards calcium extraction diminishes, resulting in extraction of other elements such as Fe and Mn that can impact mass-based yield calculations.

On this basis it was expected that one-step calcium carbonation process yields would be largely independent of lixiviant stoichiometry. To confirm this, a series of experiments (experiments 21-26) were performed using varying lixiviant molarities at a constant slag particle size (106-500 μm).

FIGS. 14-15 show that the molar ratio between the ammonium salt lixiviant and reactive calcium had no significant effect on the resulting carbonate yields above a certain minimum value during the one-step processes, even when substantially sub-stoichiometric amounts of lixiviant species are used. Only at very low values (0-0.01 mol NH₄Cl/mol CaO), i.e. in case of almost pure distilled water as a solvent, was the calcium carbonate yield reduced. Surprisingly, from FIGS. 12 and 14 it is also apparent that the carbonate yield from one-step experiments was still higher (0.25 g/g) without lixiviant than from two-step experiments utilizing a high lixiviant concentration (0.10-0.17 g/g). In some embodiments at least 5%, 10%, 20%, 30%, 40%, 50%, 60% 70%, 80%, 90% or more than 90% of an alkaline earth (such as calcium) can be recovered from a raw material by such single step processes in the absence of an added lixiviant. Without wishing to be bound by theory, the inventors theorize that carbonate formation within the raw material slag particles acts to disrupt the particle structure, thereby exposing more alkaline earth for additional carbonation.

FIG. 16 shows the recorded pH values in two-step experiments 11-13 with Ruukki slag. Extraction and carbonation are combined as one graph, even though the slag residue was filtered from the solution after 30 minutes. The variations in ammonium/calcium ratio affect the pH level; with smaller particles (which presumably contain more accessible calcium), a pH of roughly 10.5 is reached during extraction, compared to 9.7 with the larger particles. The pH decrease during carbonation also requires a longer time for small particles.

Typical pH changes during a one-step reaction are shown in FIG. 17A, where different steps in the procedure are indicated by numbered arrows. In step 1, the slag raw material is added to water to make a suspension. The amine-containing lixiviant (for example, NH₄Cl) is added in step 2, and the initial application of CO₂ gas occurs in step 3. Note that the raw material suspension is strongly basic, and that the pH drops rapidly as CO₂ is applied. The application of CO₂ is halted in step 4 and started again in step 5. Step 6 marks the termination of the reaction. Differences in pH measurements between the various experiments were small; larger amounts of reactive raw material were observed to result in larger pH changes during all time periods. A more detailed study of pH changes during application of CO₂ in a single-step process is shown in FIG. 17B shows a comparison of pH change deviations during the first carbon dioxide feed period in reactions performed at different lixiviant ratios and with different sources of raw material. At low (0.01 mol NH₄Cl/mol CaO) ammonium chloride concentration the pH changes at a slower rate than at relatively higher (0.2 mol NH₄Cl/mol CaO) for the same size fraction of U.S. Steel slag. With Ruukki slag the pH decrease is monotonic, while the U.S. slag causes an increase in pH, occurring 3-5 minutes after the start of the gas feed, possibly indicating a biphasic reaction.

The one-step steel slag carbonation process has a number of advantages compared to the two-step approach. Because the lixiviant is used in smaller amounts, the chemical cost is remarkably reduced even without recovery and recycling of the ammonium salt solution, resulting also in a simpler process setup. In addition, the higher efficiency of the process reduces the amount of raw material that needs to be processed. As shown in FIG. 18 (which shows the difference in slag amount to be processed for production of one of ton calcium carbonate with one- and two-step methods), the prior art two-step process requires 3.1-4.5 tons more steel slag to produce 1 ton of carbonate product than is required for the one-step process. This is essentially because, firstly, slag is processed and utilized to a larger extent, leaving a smaller amount of residue for waste handling. In addition, up to 140 kg more CO₂ per ton of slag can be captured in processing by the single-step process compared to the traditional two-step process (depending on the slag particle size). As such, the climate change mitigation potential is noticeably increased. Finally, if the product is a calcium carbonate that is utilized in steel manufacturing more calcium can be recycled at the steel plant, thus reducing the need for virgin calcium raw materials.

It should be appreciated that the methods, systems, and compositions described above can be equally applicable to the processing of alkaline earth elements other than calcium, for example beryllium, magnesium, strontium, barium, and radium. Similarly, precipitating compounds other than carbon dioxide can be used in methods, systems, and compositions of the inventive concept, provided that they generate suitable precipitates on reaction with alkaline earth elements. Examples of suitable precipitating compounds include sulfates, phosphates, and chromates. It should be appreciated that multiple iterations of the processes described above can be applied sequentially or in series with suitable precipitating compounds in order to generate a series of insoluble alkaline earth salts from a single source material, and that individual members such a series of insoluble alkaline earth salts can have different alkaline earth element compositions.

It should be apparent to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the spirit of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced. Where the specification claims refers to at least one of something selected from the group consisting of A, B, C . . . and N, the text should be interpreted as requiring only one element from the group, not A plus N, or B plus N, etc. 

What is claimed is:
 1. A method for the recovery of an alkaline earth from a particulate raw material comprising; providing the particulate raw material in the form of a mud or slurry in a reactor, wherein the particulate raw material comprises an alkaline earth; in the same reactor, exposing the raw material to a source of carbon dioxide at ambient temperature and pressure, thereby generating a first reaction mixture; during contact with the source of carbon dioxide, monitoring pH of the first reaction mixture; discontinuing contact with the source of carbon dioxide when pH of the first reaction mixture becomes less than 8; monitoring pH of the first reaction mixture following discontinuation of contact with the source of carbon dioxide; re-establishing contact with the source of carbon dioxide at ambient temperature and pressure when pH of the first reaction mixture exceeds 8 to generate a second reaction mixture comprising an alkaline earth precipitate, and an extracted raw material; and separating the alkaline earth precipitate from the extracted raw material, wherein the method is performed in the absence of an added lixiviant.
 2. The method of claim 1, wherein the alkaline earth precipitate and the extracted raw material are separated on the basis of particle diameter.
 3. The method of claim 1, wherein the alkaline earth precipitate and the extracted raw material are separated on the basis of at least one of particle characteristics selected from the group consisting of particle density, particle shape, and effective particle hydrodynamic radius.
 4. The method of claim 1, wherein the separation is performed using a centrifugal separator.
 5. The method of claim 1, wherein the separation is performed using a filter.
 6. The method of claim 1, wherein the separation is performed within a portion of the reactor that is configured as a settling tank.
 7. The method of claim 1, wherein the raw material is introduced into the reactor in an essentially continuous manner.
 8. The method of claim 1, wherein the precipitant is introduced into the reactor in an essentially continuous manner.
 9. The method of claim 1, wherein separating is performed in an essentially continuous manner.
 10. The method of claim 1, wherein the extracted raw material is subjected to further processing.
 11. The method of claim 1, wherein the alkaline earth is calcium.
 12. The method of claim 1, wherein the raw material is selected from the group consisting of steel slag, lime, dolime, acetylene waste, and municipal ash waste.
 13. The method of claim 1, wherein at least 50% of alkaline earth content of the raw material is recovered as the alkaline earth precipitate. 